As is known, many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by creasing and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; the web so sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed vertically.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections.
More precisely, the tube is sealed longitudinally and transversally to its own axis.
Pillow packs are so obtained, which have a longitudinal seal and a pair of top and bottom transversal seals.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the packages are then filled with the food product and sealed. One example of this type of package is the so-called “gable-top” package known by the trade name Tetra Rex (registered trademark).
A package is known which comprises:                a rectangular bottom panel which is crossed by a bottom transversal seal;        a rectangular top panel, which is crossed by a top transversal seal;        a rear panel which extends between corresponding first edges of top and bottom panels;        a front panel which is opposite to the rear panel and extends between corresponding second edges, opposite to first edges, of top and bottom panels; and        a pair of lateral panels interposed between bottom and top panels, and between rear and front panels.        
A longitudinal seal extends perpendicularly between the transversal seals and along the centerline of the rear panel.
In greater detail, rear and front panels are parallel to one another, lateral panels are parallel to one another, and lateral panels are orthogonal to both rear and front panels.
Furthermore, the top panel is slanted with respect to the bottom panel and is descending proceeding from the relative first edge to the relative second edge.
In other words, the distance between first edges of bottom and top panels is greater than the distance between second edges of such bottom and top panels.
As a consequence, the height of the rear panel is greater than the height of the front panel.
The top panel also comprises:                a first area which accommodates an opening device and is bounded, on opposite sides, by the second edge and the transversal seal; and        a second area which is bounded, on opposite sides, by the first edge and the transversal seal.        
More precisely, the opening device substantially comprises a frame fitted to the first area of the top panel and a cap which is releasably coupled with the frame, so as to free a pouring opening of the food product, when unscrewed.
The above type of package is known under the name Tetra Brik Edge®. Its main characteristic is the slanted top panel, which brings two advantages: an angled top panel which improves the pourability, and a transversal seal, crossing the top panel, being offset towards the rear panel, leaving a greater area for placing a cap.
Such a known package could be uncomfortable to pick up, especially when it stands between several other similar packages on the shelf, or in a secondary package such as a cardboard box.
Furthermore, in correspondence with other carton packages, the reduced circulation of air between the packages, as a consequence of being tightly packed, could cause the formation of moisture onto such packages, especially if packed in a closed environment, such as a plastic wrap or a cardboard box, and even more so when subjected to humid weather conditions.
Due to the height of an opening device and to the inclination of the top panel, the opening device may protrude from the first area of the top panel beyond a hypothetical prolongation of the front panel as shown in FIG. 1. Since consumers today want bigger caps, for an improved drinking experience and improved pouring performance, this problem will only increase. Furthermore, newly developed one-step opening caps are higher than previous two-step opening caps, and this means that the caps stick out even more outside of the front panel. A one-step opening cap is a cap that only requires one action from the user, such as unscrewing the cap, in order to prepare the package for pouring or drinking.
A group of packages is often stored and transported in boxes as shown in FIG. 2a. In this condition, the front panel of a second package contacts the rear panel of a first package. Furthermore, the front panel of an initial package and the rear panel of a last package are in contact with respective inner walls of the box.
Furthermore, the opening device of the second package interferes with the rear panel of the first package and therefore exerts a pressure against this rear panel.
Accordingly, there is a risk that such pressure causes the opening devices to come off from the relative packages. The opening device could also cause a dent on the adjacent package, which causes an unwanted appearance and could make a customer reject the package. The dent could in worst case also affect the integrity of the package.
Furthermore, the interference between the opening device of the second package and the rear panel of the first package tends to slightly detach the front panel of the second package from the rear panel of the first package.
Accordingly, the rear panel of the last package and the front panel of the first package are forced against the walls of the box.
As a consequence, there is a risk of damaging the packages within the box or the shrink wrap, or even to cause damage to the secondary box or shrink wrap itself.
Finally, packages with opening devices applied thereon are normally transferred on a belt conveyor within the packaging plant.
In case that the conveyor is stopped or packages are grouped, a queue of packages with opening devices is formed. This happens relatively regularly in the packaging line.
In this situation, the packages are pushed against each other, and can become slightly angled or leaning with respect to the conveyor as a consequence of the interference between their opening devices and the rear panel of the adjacent package, as shown in FIG. 2b. 
As a result, there is the risk that one or more of the packages fall over on the conveyor. This can cause a jam and the filling line must be stopped to fix the problem. It is also possible that some packages fall off the conveyor, due to this inclination, leading to waste of packaging material and product.
Furthermore, a need is felt within the industry to easily detected the fermentation of the pourable food product within the package and/or the leakage of air (or another gas) into the package.
EP-A-277673 discloses a sealed package comprising a quadrilateral top wall, a quadrilateral bottom wall, front and rear sidewalls and lateral sidewalls. The top wall is not provided with an opening device.
The joint portion between each two adjacent walls of the front, rear and lateral four side walls is formed with two ridgelines interconnecting the corresponding pair of corners of the top and the bottom walls.
The front wall extends between the front edges of top and bottom walls. The rear wall extends between the rear edges of top and bottom walls.
The two ridgelines are joined to each other at their upper and lower ends but are spaced apart from each other between these ends. The ridgelines extend smoothly as curved inwardly of the package to form a joint wall therebetween.
WO-A-2009/030910 discloses a package comprising a top and bottom walls, a front and a rear wall. And two lateral walls.
Top wall is sloped relative to sidewalls and to bottom wall. In a first embodiment, the front edge of the top wall—which coincides with the top edge of the front sidewall—is curved.
In a second embodiment, the package comprises a recess for receiving the nose of a consumer drinking directly from a spout. The recess is formed by providing a lenticular panel in the sheet packaging material from which package is formed.